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Full text of "The Introduction of Packet Satellite Communication"

10/25/95 
09:15 
''703 620 0913 
002 
--.' TME INTRODUCTION OF PACK-T SATELLITE COMMUNICATIONS 
Robert E. Kahn 
Advanced Research 
Abstract 
The preparations which led to the SATNET 
xeriment are discussed in this paper. Various 
packet satellite tariff considerations and 
architectural issues are presented along with a 
smmary of future plans for use of the technology. 
1. Introduction 
This paper reviews the process' which led to the 
introduction of packet stellite technology in the 
970s, The development of this technology was 
undertaken by the Advanced Research Projects 
Agency (ARPA) in order to evaluate its utility for 
efficient long haul computer communications with a 
potentially large number of geographically 
distributed users. TBis effort was undertaken in 
conjumctioq with participatimg organizations in 
the O.K. and.Norway, but does not necessarily 
reflect their ¾1ewa on this subject. 
The%most notable example of this technology is the 
Atlantic Packet atellite Network, known as 
SATNET, which has been in operation on the 
Atlantic Intelsat IV satellite since late 1975 and 
which currently serves a community of researchers 
in the U.S, the O.K. and Nory. Underlying the 
SATNET technologF is the basic packet switching 
technology which wa first introduced during the 
late 1960's. The November 1978 IEœ Proceedings 
contains a comprehensive treatment of packet 
communications technology and includes a paper on 
General Purpose acket Satellite Networks which 
provides a good introduction to the subject [I,2].' 
SATNœT consists of a__i.ng_e bad%ast channel 
shared by multiple er%h tions w'nlch use Time 
Di¾ieion Multiple Access (TDMA) and emit packets 
according to a channel access protocol. Tese 
earth stations may be connected to one or more 
aubsoribem nctwor.. Eeh earth station contains 
& prorable satellite processor (a controller 
and related electronics) ich implements the 
satellite channel protocols and interfaces. The 
system provides complete connectivity between all 
the participatin& earth stations and allows 
dynamic allocation of the satellite channel amens 
them. Different priority levels ma be supported 
efficiently on the same channel without 
unnecessary preallocation or preemption of 
Projects Aency 
resources. Tc broadcast property of the channel 
enables a transmission from one earth station to 
be received by 811 the others including itself. 
Both conferencing and delivery of multiple address 
packet ca be achieved efficiently as a result. 
The Arpanet was the first example oF a packet 
switched network which used point-to-point 
terrestrial lines (across the U.S.) in a store anJ 
forward sFstem [3,]. Following the installation 
of the first Arpanet nodes, a number of papers 
appeared in the literature on the application of 
packet switching to multiple access radio'channels 
[5,6,7,8]. The ARPA-sponsored effort at the 
University of Hawaii wes the first to emonstrate 
burst transmission oF packeLs by radio for 
computer access by terminals within line of sight 
of the computer center. In this system, called 
the ALOM system [9,10], packets were simply 
transmitted when they were ready to send - at 
random instants of time. No explicit control of 
the radio channel was invoked. Rather, o 
occasion, packets would collide in the air, 
destroying each other and would be retransmitted 
t a later random time. The multiple access 
nature of this system resemble a packet satellite 
net, except that the terminals were much closer to 
and quite unequally spaced from the computer 
center which (like a satellite) formed the hub of 
the system. The Hawaii researchers extended the 
concept of radio packets to satellite 
communication directly, and experimentally 
verified the concept using test packets over 
NASA's ATS-I satellite between Hawaii and 
NASA-Ames. The technique of operating a Packet 
Satellite Net in an uncontrolled fashion became 
kno aS "Pure Aloha". 
A significant body of theoretical wor on the 
analysis of Aloha Systems appeared in te 
literature in the early 1970's and various 
improvements on the original random transmission 
technique were proposed and evaluated [11,12]. In 
the Slotted Aloha technique, first introduced by 
Roberts, the time axis at the satellite is divided 
into equally spaced intervals called time slots 
which hold a single packet [133. Under the 
Slotted Aloha regime, packet an only be 
transmitted startin& at the beinning of a slot. 
For fixed length packets and PoXseen traffic 
arrivals, the capacity of the slotted system is 
twice that of the unslotted system due to the 
reduced number of collisions at light traffic 
------------------------------<page break>-----------------------------
10/25/95 09:17 '703 620 0913 
OO3 
loads, In oth the slotted and the nslotted 
ease, Ome form of stability cntrol is eeded 
(1,15]. For efficient use of a packet satellite 
channel, where the cne-way-ranslt time is much 
of satellite channel allooatlon strategy i 
appropriate [ 1O]. A priomlty oriented demand 
s].1oea%lon of oafcity and ls orently In aily 
use In STNET 
tb simulation and analysis were used extensively 
d effectively in vestigai ese mnd other 
cbnel access sches. -ve, thim r s 
able to deal effectively on a purely theoretical 
associated with develoent of e tecnnoloEy. 
Access to an exrentl system s essential to 
address topics such as fault etection and 
iolation, system stat monitorln and debin&, 
terfacl to terretri netr and atewas, 
ftware structure d rfoanoe. It was 
feasible to oarr out a test of the tecnnolo&y on 
e of several existin stellites a it apare 
as if exlstl grod stations could be ed  a 
cet mode of operation th only 
mifications to provide external on/off control 
of the carrier by a prorable satellite 
processor at each earth station. e cket 
satellite technology s also seen as a 
tentlally useful lon& term adjunct to exittin& 
netrk tecnolo6y for lon ha applications 
vclvint oonfarencin&, mti-destiation 
broadcasting d escially to provide 
connectivity between a lar&e nr of sites (each 
with low duty cycle traffic) usin& a 11 
fraction of %he leased channel banddth that a 
fully co.acted actwork of polut-to-t olrouits 
uld have required- 
In the 1973-1g? te frame, the only viable 
choices for such a test (fr the U.S. point of 
view) were the Intelsat satellites, the NASA 
rental satellite and one of the several 
military satellites. The Intelsat syat s 
preferred choice for this activity ecae It 
uld be made available most easily and had the 
tential for supporting the reultl tecology 
on s ceial basis upon cpetlon, if it 
proved to be econic. The military satellites 
re less appropriate ohoices as tere s not yet 
a stated requirement for cket satelll service 
in the military. At$.te, international 
terest in packet switCi s growi 
significantly, and ssible requirements for 
interconnection of domestic cket networks in the 
different co.tries were identified. In 197B, %he 
PANET ad Just been extended to orway and the 
U.K., and expertentel use of the APANET was 
provint to e quite worthile for resesrc 
purposes. 
is is the context in which the subect of an 
exrental prr on cket satellite 
tecbnolo6y was first raised with the British PoSt 
office, with the Communications tellite 
rration (Comsat), and subsequently with 
Norwegian Telecommunication Administration (NTA) 
-end .Noulan Defense eaearch Stablisment 
,,(DBE)...Jm the follong section, 
preparations for te SAET exrent arc 
tlin al with the approval proess icb s 
2. Preparln for te SET rent 
In 97, &he U.E.-Poa Office ree o support 
the SATNET exrent by contributing the O.K. 
half of te satellite link and by providing access 
 the necessary eart.mtalon eqent in 
gland. A progrle mtelltte processor was 
installed ab te Goonhilly earth station and 
ected back tca Eatery at the ARPANET node on 
te praises of University 11ege ndon (OCL) 
with a 8 Kbps oicatlon line. UCL was 
prepared to accept the main research 
resnsiility for the O.K. prticipation l te 
SATNET progr, an subsequently di o. 
Also in 197, sat agreed to U.S. participation 
In suc  expertentel activity, ut only if 
We carried out under te auspices of one of the 
several U.. International Record arriers (IRCs) 
icb historically have played the le of 
inteediary in ringlng lnternatin data 
services to the end customer. omsat is the U.S. 
presentative to Intelsat. en te SATNET 
project was bei formulated, Comsat also operated 
otb the space segment er contract to Intelsat 
d the .S. earth stations for the conrti of 
U.S. oers. Intelsat itself now orates 
space segment. 
The only generic classes of service wlch could be 
offered by Comsat were those specifically approved 
(tariffed) by Intelsat. Clearly, e cket 
satellite service was not onE the. After 
rlod of iscussion within telsat lasting 
saver monts, an Intelsat tariff for a 
multi-station service was approved In late t97. 
e SA progr was initiated in ptber 1975 
with one Intelsat stdar A ( meters) earth 
station at Eta, West Virginia and a silar one 
at Gnbilly s, gland. Within Noy, the 
interactions wit the NTA were handled entirely by 
te Norwegian fense esearcb Estblisent. 
ile Norgtan partiolpation In the SATN 
progr had started with the first meeting of 
searchers In 1975, their active rticipation on 
the channel began  late 1977 usi the Nordic 
earth station at Tan, Sweden. Shortly 
tereafter, msat Laboratories mae preparstlons 
to rtlcipate actively with a small nattendeO 
rtb Terminal (ET) at larksburg, ryland for 
system diandais and evaltion. e PET differed 
fr the three standarO A earth stations In that 
it bad only a 10 meter antenna and could only 
ceive at 16 Kilobits/second lle the other 
stations could receive at 6 Kilobits/second. All 
four stations can transmit at 6 Kiloite/second, 
but the large stations must duce their 
transmission rate to 16 Kilobits/second to talk to 
te T. 
------------------------------<page break>-----------------------------
10/25/95 09:18 '703 620 0913 
The technical aspects of the SATNET experiment are 
not addressed in this paper. Other companion 
papers address both system level and experimental 
aspects of the program ['19,20,2]. In the 
remainder of this paper, the relevant tariff 
considerstigma will be diacused and two key 
architectural issues will also be considered. 
3. Intelsat Tariffs 
The new Intelsat tariff which was approved in late 
1974 was for a new kind of service known as 
Multi-Destination Half-Duplex (MDHD). Simply 
stated, MDHD allows one r more members of 
Intelsat to Jointly share a eon channel on any 
of the Intelsat Satellites for a modest MOHD 
payment to Intelsat. The normal leased service 
offerings from Intelsat to its members are a 
point-to-point service between two earth stations 
and a broadcast Service from one prospecified (but 
fixed) earth station to at least two others. The 
point-to-point service can be either half-duplex 
(one way) or full-duplex (two-way). The broadcast 
service utilizes only one channel, as a reverse 
path is not included. 
The MDND capability may be viewed as an extension 
of the broadcast service %o allow moe than one 
prospecified earth station to transmit. MDHD 
allows all participating earth stations to 
transmit using their own channel access protocol ' 
to resolve contentions. 
To any member country already participating in an 
MDHD service, the added cost is nominally zero 
allow additional earth stations to share the MDHD 
channel. This assumes that capacity limitations 
'are not exceeded and that coordination among a 
larger number of sites costs the same. Mwever, a 
payment must be made to Intelsat by each member 
country which chooses to Join (share) an existing 
FHD channel, so the total payment received by 
Intelsat for MDHD service grows linearly with the 
number of countries. The easons for a choice of 
tariff in which the cost per country is 
independent of the number of participants depends, 
in part, upon the olitical structure of Intelsat. 
The subject of PTT tariffs to the end customer, 
although not specifically discussed in this paper, 
would generally include earth station 
terrestrial charges, as well as space segment 
charges. 
If we assume Intelsat'normally charges amember C 
per one-way channel of a certain capacity for a 
total of 2C counting bth ends, then the same 
revenue would be gathered if each of the 
participating members in an HD channel wre 
charged 2C/N apiece (assuming N participants). 
The members, in turn, could base charges to their 
customers on these costs plus the added costs of 
ground station euipment and terrestrial 
interconnection. This kind of formals in which 
the space segment charge is independent of the 
number Of earth stations appears well-suited to 
domestic services where all the earth stations are 
owned by one authority. However, this formula 
poses several problems when applied to the 
,international situation,.where the earth stations 
are separately owned and operated. 
First, the eats ase ½r aeh participating 
ountry ould fluctuate as ß function of the 
number of participating countrieS, making 
fi&ncial management nd planning awkward and 
unpPedictable at est. .Second and mute 
imprant, .le voing rights f eaah member 
country in Intelsat are a function of its total 
ayments to Intelsat. Primarily, for that rason, 
the Intelsat Mp tariff ws fixed to be a 
costant ½ per channel per country. 
Te Intelsat broadcast tariff illustrated in Fig. 
1(a) shows one transmitter which is charged C and 
four receivers each of which is charged C/2 for a 
total of 3C. Since at least two receivers oust be 
present for a broadcast service, the minimum. 
charge is ZC (wich is identical to the half 
duplex in-to-plnt tariff between two 
The revenue produced by the broadcast tariff 
increases linearly with the addition of more 
ground stations at an increment of C/ per added 
receiving station. 
The MDND tariff illustratcd in Fig. l(b) shows 
each participating country being charged C for the 
right to receive add transmit on the same channel. 
The net payment to Intelsat, 5C, is almost double 
the charge for the smple broadcast case. 
However, the value received for this added cost is 
full N-way cOnneCtivity since anX of the earth 
stations can transmit to the others at anF time 
according to the chosen channel access protocols. 
The MDHD tariff is also considerably cheaper than 
that for N distinct broadcast channels to 
implement N-way connectivity (NC va. INC  
N(N-1)C/2]). Along with the initial hiher cost 
of N boadcast channels would nome N tes the 
capacity, however, eEardless of wether It ess be 
ud effeotively or not. 
With these existing tariffs, the cost per country 
normalized by total number of channels of network 
capacity available o the N earth stations is C 
for the I'HP case and [C + (N-1)C/]/N = (N+lJC/2 
for the case of N broadcast channels. If existing 
MDHD tariffs are extended to channels wit a 
higher bandwidth using a 'linear extrapolation" of 
the current tariff, the charges for obtaining the 
added capacity with multiple lower capacity 
broadcast channels will be half as much as the 
single MDHD channel as the number of participating 
earth stations becomes large. Since this ratio 
eflects only the current tariff structure, the 
ratio could be changed (e.g., becoe closer to 
unity) with a non-linear tariff revision 
applicable to higher bandwidth channels. 
From an architectural point of view, the use of 
multiple roadcast channels has both positive and 
negative features which are identified in section 
6. However, in most applications, it is doubtful 
whether initial network-wide traffic will be large 
004 
------------------------------<page break>-----------------------------
10/25/95 09:20 "703 620 0913 
005 
enough tc justify commencing service with more 
than a single MDD channel. 
4. COMSAT and IRC Filings in the 
The U.S., U.K. and Norway participation 
has been on an experimental basis and a service 
has not yet been offered to Customer in any of 
eae countries. In the U.S., aa filed a 
rff with the FOC in 1975 %o offer - 
exrimental acke% atelllte cability 
 its desinateO =oatmatters via one of the 
ICs. The service sat offend was based On the 
 servie obtaln fr Intelsat, d 
aented as requid with the DroErable 
satellite pocessor at the eart s%ion. In its 
filing, sa also eferred to its 
service as HD. 
sat bought or leased all te necessary Kroud 
tation equileat to provide the exrental 
service as for a nodal oeroisl 
offering. In a competitive selection, Western 
Union International (WUI) was cnose b the 
service to the ARA proEr. WUI, in tn, filed 
obtained fr Comsat, which they amented witD a 
terrestrial circuit before supplying it 5o the 
6over. eat. 
goVeret's request for proposals was umual in 
that it did not dire any specific destination or 
custe location ahead. Rather, 1% sply asked 
for  HD channel from the U.S. o an 
pecified int in the U.K. and sated taC all 
of the U.K. costs re to  as$ed by the 
British Pot Off lea. A lnt of contact In 
Post Office was identified. e request also 
stated that additional mpecified destinations 
mh% ave to e connected subsequently, as Nervy 
eventually was. 
To validate te initial delivery of the service 
and to verify meatoral of service in he event 
outage, only a loopback test fr the 
cuzter site (ich was speclfle to  the 
iic Data alysis nter tn Alexandria, 
Virginia) to he satellite and ack wa require. 
The paten% to WUI was not cndent on the 
partioilon (or rfoanae) of y other 
co.try (or its equ..owver, the  and 
COIAT tariffs both included a small charge 
ortional to the nr of participating sites 
for cochinatica. 
A diar of the HD paent flow during the 
experiment Is sho in Fi. . 
5. SATET rent 
The SATNET ent was conducted nlnally 
dui the period from ptemr 1975 through 
ptember 1978 and involved researchers fr each 
of the three rticiti countries. The basic 
physical architecture of SAET was dictated by 
many programmatic considerations (e.g. ue of 
existing ground stations and satellites) so the 
actual hardware configuration merely reflects what 
%rag available for use in the experiment. However, 
tDe logical architecture of the system has been a 
auJeot for research and bas constantly evolved 
during the coUrse of the program. Neither the 
software architecture nor the system protocols 
were prescribed in advance and the aSh-hardware 
parts of the system interfaces were also allowed 
to evolve, which they did. Each was e major 
subject for investigation and exploratory 
development during te CoLirma Of the project. The 
resulting logical architecture wll be ¾ery useful 
in designing a more advanced follow-on system. In 
addition, an effort ws undertakem to develop and 
demonstrate a high prformance digital burst modem 
and error control unit for possible operational 
use wit SATNET after the experiment. 
A major decision in the program was to separate 
the SATNET development and testing from the 
closely related internerring research activities 
which were Just getting underway. It was decided 
to pursue the internerring research using a 
separate minicomputer gateway in each country 
simultaneously connected as a Host on SATNET and 
as a Host on the Arpanet [22,23]. This 
arrangement left enough flexibility to pursue 
gateway related research without resulting 
software changes (in rel~time) to SATNET or 
Arpanet. The gateway software could have been 
incorporated within the physical confines of 
either SATNET or Arpanet, or split between them. 
owever, keeping it separate for the purposes of 
the experimental program provided maximum 
flexibility to the internettln s researchers, many 
of whom were also working on SATNET, Arpanet or 
other ongoin networ related pro, rams without 
unnecessarily distracting those SATNET researchers 
who did not need to be deeply involved in the 
internerring work at that time. 
Technical direction of the program beginning in 
September 1975 was the responsibility of Linksbit 
Corporation, San Diego, allfornia who prepared a 
comprehensive test plan to guide the conduct 
the experimental program. Major participants were 
CO. sat, Bolt Beranek and Newman, University of 
California at Los Angeles (UCLA) and the Defense 
Communications Agency in the U.S., University 
College LOndon and the Post Office in the 
and NDRE and NTA in Norway. 
Coordinating a program involving participants fro 
multiple countries as an important challenge that 
was met at several different levels. Ouarterly 
review meetings were held (otated among the 
different locations) and attended by all the 
participants. Technical progress was reviewed at 
these meetin&s, technical issues were discussed 
and resolved and plans for each succeeding usrter 
were refined. Research issues end results were 
docented and circulated in a series of informal 
working group notes. The ARPANET played a 
particularly important role in executing the 
effort as well as in coordinating it. It provided 
------------------------------<page break>-----------------------------
10/25/95 09:21 703 620 0913 
006 
ira means by which %he satellite processors were 
down-line loaded and debugged, and the meas by 
aich SATNET itself was cTtolled and monitored 
as i% was being developed. The message pasir 
capability of the bo3ts on the ARPANET were used 
to keep all participants informed of technical 
progress, system status, often by direct reporting 
from tl%e programmable satellite processors in 
SATNET, and to resolve questions and culminate 
experiments on a day-by-ay basis. Without much a 
capability, it is doubtful that the overall 
experimental program could have been carried out 
successfully. 
The main results of the exriment are bainõ 
documented by Linksbit Corporation (with inputs 
frm all the participants) in a final report to be 
available shortlyl A Smmary of the findings show 
that the SATNET experiment demonstrated the 
feasibility of the packet satellite technology, 
llluminaed many of the most iportant technical 
and non-technical issues and provided a system 
that can support advanced co-purer comunication 
research applications. lthouh the subject of 
packet voie has not been emphasized in this 
article, it played an integral ole in the SATET 
design. SATNET is the only operatin lone haul 
packet switched network in the world that has been 
demigned to handle both packet switched voice and 
data. 
6. Architectural Issues 
Two architectural issues arose durin the Course 
of %his project which are appropriate to single 
out for mention. The first issue is selecting the 
functionality that ought to reside in the 
proccssor which is cOldCared with the rest of the 
earth station equipment and the functionality that 
ought to reside at the terrestrial interface {to 
the earth station) which might be located some 
distance from it. The second major issue concerns 
the means of increasing the overall traffic 
handlin E capacity of the system. Each of these 
issues are briefly mentioned below. 
a. Functionality of the Earth Station and its 
Terrestrial Interface 
Although not all the functions mplemented in 
SATNET need to resi__-.h earth station, a 
minimu set of functions must be located there to 
control timin and access to (and transmission on) 
the satellite channel. Barts of the functionality 
might he moved to a terrestrial location distant 
from, but connected to, the earth statio by a 
communication line. One attempt at the definition 
of the functionality is iven in [2]. In 
particular, certain aspects of the functionality 
which deal with multiplexE traffic r many 
'uers into a comslte stream to the earth station 
could proably be relocated without penalty in 
erformance provided delay or unrelitbility is not 
added outside the earth station. Accountin and 
other administrative functions could also be 
remo%ed from the earth station without penalty. 
b. Expansion of Network Capacity 
Although a single 64 Kilobits/second channel is 
:learl uld be nufficient for many 
applications. The capacity of a SATNET system 
could be expanded l several ways. First, it 
could be simply scaled up in data ate. The 
ability of s packet switch to handle. 
ultl-eabit/econd data as been demonstrated 
[]. Ibis would suffice for an expansion of one 
or two orders of mEnitude. A transponder can 
typically handle upwards of 6 4eEbits/second , 
however, nd the newer smtellite systems are 
epected to support higher data rates still. 
Multi-Droeemsor systems see capable of supportin 
these hi,her data rates on a single shared 
satellite channel without either increased delay 
in bufferin or processir. owever, the number 
of processors must grow linearly With capacity and 
special attention must be paid to communication 
between processors and with external devices. 
A second alternative, whic baches attractive 
wen the overall netrk traffic is high enough, 
is to corpoate dedicated upliks using 
Frequency Dlvfslon Multiple Access (FA). 
this scheme, which is illustrate  FiE. 3, a 
separate Drocesso at each eart station uld 
dolink and would pa=s alons %o a concenta%or 
only hose packets elned fo i earth 
Te capaelty of the concenao ould %hen be 
sized  he thoushpu inSended fo ha site 
wlch 6sably d be h les than th total 
network traffic. In this scheme, a mification 
would be required at each exlatin E Eund station 
fo each new addition to the net, which is a ma3om 
isadantage. However, it s highly modular and 
shoed be easy to upErmde in an orational 
system. 
The use of multiple FA broadcast channels, one 
per site, reUuoes the earth ttion prccessinE 
requirements but it also does not provide the 
flexibility that comes from the dyaic 
of pacity in a MA system. A thir alternative 
is a hybrid of cases one a two above in which 
e of the uplin may be BD channels (using 
TA) ile the rest may be badcast channels 
each from a sidle source. 
7- Future Plans 
SATNET currently sees as the backbone for a 
r of innovative research applications d has 
bece the prima packet transrtation vehicle 
ten the U.S. and rope for cputer 
lcations and command and control research. 
SinCe  1979, AANET access fr the U.K. has 
r lmost exclusively via SA on a provisional 
sis. It Is planned to continue the use of 
SATNET as the primary link between ARPA end its 
search partners in rope. e ARPAN link 
London (via Norway) is scheduled to be taken 
during the last quarter of 1979 aer which 
only available ARPANET access fr the U.K. will 
------------------------------<page break>-----------------------------
10/25/95 09:23 '703 620 0913 
 007 
b via SATNET. NDHE will utilize SATNET for 
research purposes; %he only planned use of the 
rmanir pOin-to-point ARPANT link fro the 
U.S. to Norway will be foletrieval of seismic 
data, which was the o/6inal function of that linc 
prior bo its incorporation in the ARPANST in 1973. 
WithiD the U.S., ARPA plans %0 use the SATNET 
satelite =hannel operatir a 3 Mbps with 5 
anteana initially at Lincoln Laboratory,. i 
xtton, saachuetts d USC/ISI in rina 
ey, Califomma. AdditOn sites In the n 
Fansieo aea and Washi.tun, D.C. aea will 
also be adoe. A o uae fo %he 
channel is o exploe ne use. of the ATNET 
techn01oy %o supp% mul:i-use lnte6ated packet 
voice nd Oatm emlCation. ly with the 
increseed anwidth will a test of this concept 
ssile usin multiple aee an dea sources 
including a mix oF 2.4 Kilobits/second to 64 
facsimile and nodal cpu%e %o upu%eF 
The fense iationa eny also plans 
utilize tRis tecolosy alon with ARPA for 
advanced re.arch and develoet on D 
tegrated sta/voice networks of the ture. 
 the international soene, packet satellite 
technology may be ueful fo a wie variety of 
ten%ial applications. e such possibility 
ich is bein E offered as a seviee by the 
and the US Postal Sewlee is Intelst. This 
an innovative new facsile service ich is 
Dlan evolve, inividual point-to-point links 
oory. A cke stelite y could support 
=he initial Intelpot raffie %h oly oe shaed 
channel, wit consideal less oal aellic 
owidh th multiple point-tit link would 
quie and without noticeable de6radaion of 
Acowledmenta 
Tis effort would not have been ssible without 
e eooera%on and support of e BPiih 
Office an the Norwegian Teleeiatlons 
Admiustration;'  orgai:ios layed a ery 
central le In he Drr. lt ranek & 
Nean (RtN), COMSAT, an LinCbit rporation 
played signifil,mr _zz!n evepi th pak 
satellite technology. CT speahesded the 
sppval priest. UCL, NDRE, UC, and 
with the assistance of BN, caied out the 
decDire %he lapse ea9hic distan=es fF SATNET 
and each other which might otherwise have been a 
deterrent. The success of the progr was due 
no mall measure to e technical direction 
provided by Linksbit rporation. 
He f orenoes 
[1] Special Issue on packet Comunications, IEEE 
Proceedings, Nov, 1978 
[] 
I.M. Jacobs etal, "General Purpose Packet 
Satellite Networks," IEEœ Pc., pp. 1418 - 
1467, Nov. 1978 
[]] 
L.O. Roberts and B. U. Wessler, "Computer 
Network Development to AChieve ReSource 
Sharing," AFIPS Conf. Proc., $JCC, pp. 
[] 
R.E. Kahn, "Resource Sharing Cxxuputer 
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2 2 
SiTE 1 IC/2 SITE I 
Fig. 1. (a) Broadcast Tariff. {b} HDHD Tariff 
NOIVAY 
Fi. 2. MDHD Payment Flow 
BROADCAST DOWNLINKS 
BROADCAST 
UPLINK 1 2 3 N 
P--'-----'=-:'--' t CONCENTRATOR Et INTERFACE 
TO TERRESTRIAL NETWORK 
Fig.. 
Station Configuration for Multiple Broadcast 
Charmelm in a High Capacity System 
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